Closure caps including crown and container caps having a liner or packing of rubber, resin, paper or cork have heretofore been widely used as closures for sealed bottled products. Those closure caps provided with a liner of low density polyethylene or a vinyl chloride resin are excellent in that the liner has good shapability and sealing properties and does not change the flavor or taste of the bottled drink or the like. Moreover, the manufacturing costs of these closure caps are relatively small. Accordingly, the demand for these closure caps has recently been increasing.
One known process for making such closure caps comprises supplying a mass of a melt of a thermoplastic resin to the inner face side of a closure shell, pressing the supplied mass between a cooled pressing mold and the closure shell and thus forming a liner (see Japanese Patent Publication No. 5588/66). Another known process comprises supplying a preformed thermoplastic resin to the inner side face of a closure shell, heating the closure shell to soften the thermoplastic resin, pressing the softened resin between a cooled pressing mold and the closure shell and thus forming a liner (see Japanese Patent Publication No. 5706/73).
Liners should be tightly stuck to and engage with the open mouth portions of containers. Accordingly, it is preferred that one or a plurality of annular projections or concave grooves be formed on the peripheral portion of a liner to be engaged with the open mouth portion of a container.
Conventional press molding processes for the preparation of liners are advantageous in that the operation of molding a mass of a thermoplastic resin into a liner and the operation of sticking the liner to a closure shell can be performed simultaneously. However, it has been found that these liners are often inferior to disc-like packings or liners formed by punching out discs from sheets of liner material especially in sealing and adhesion properties.
More specifically, when a closure cap provided with a liner formed by press molding is used for sealing a bottled product, micro-cracks may form in the liner, especially at the peripheral portion. This cracking may result from the compression force applied to the liner during capping, from contact with the contents of the bottle or the vapor of the contents and from sterilization or pasteurizing steps. The result is that adhesion of the liner with the shell degrades over a period of time. This is particularly true with closures used with containers having a wide mouth, where the operation of the press-molding of a liner is carried out at a high speed or when the liners have annular projections or concave grooves on their peripheral portions.
It is therefore an object of our invention to provide for a resin liner formed by press-molding which will have a satisfactory sealing property, have a high peel strength and have a high resistance to heat treatment.